Gemini Observatory: Exploring the Universe, Sharing its Wonders

PHOENIX on Gemini South Reveals Clues about the Origin of Fluorine

August 19, 2003

PHOENIX was used
to sample multiple stellar atmospheres and has revealed tantalizing clues
that neutrino interaction during the supernovae explosion of massive stars
is involved in the genesis of fluorine. PHOENIX uses a Gemini supplied 1024
x 1024 InSb Aladdin II array.

Despite
our awareness of fluorine's unique properties, its astrophysical origin is
not well understood. Like most heavy elements, fluorine is a product of nuclear
reactions in the hot cores of stars during various phases of their evolution.
Furthermore, fluorine is not easy to observe in the universe, and very few
spectroscopic measurements of fluorine in stars exist. Until the recent Gemini
work reported here, only a meager set of data for a few stars in our own
Milky Way was available.

A spectroscopic
program conducted at Gemini South, located on Cerro Pachón, Chile,
has changed this picture. Katia Cunha (Observatório Nacional, Brazil)
led a PHOENIX team of researchers on Gemini South to measure the abundance
of fluorine in our own galaxy's satellite, the Large Magellanic Cloud (LMC),
and in the massive galactic globular cluster
Centauri.
The team studied fluorine in its most accessible form, hydrofluoric acid
(HF), which is detectable through vibration-rotation transitions falling
in the near infrared close to the wavelength of 2.3 microns (main figure and Figure 1). These Gemini observations
provide unique insight on how fluorine behaves as a function of the abundance
of the other elements, specifically metals, helping to identify how fluorine
is made.

To better
understand how the abundance of fluorine depends on metallicity and stellar
populations, Cunha and her collaborators compared their Gemini measurements
with known K and M type stars of our own Milky Way. The total sample of stars
with fluorine abundance determinations contains 23 red giant stars across
three stellar populations including the solar neighborhood,
Centauri
and the LMC. It is to be noted that the abundance of fluorine, like that
of other elements, does not only depend on the yield from nucleosynthesis
processes. Star formation history and element dispersal mechanisms can also
play important roles.

Figure 1. Observed
(dots) and modeled (lines) spectra for the LMC giant star 2.3256. Three
fluorine-19 abundances are presented. An abundance of A(F)=3.93 - with
the Sun hydrogen abundance being 12.0, on a logarithmic base 10-scale
was derived.

Nine red
giants stars in the LMC and two in
Centauri
were observed with PHOENIX. The behavior of the fluorine abundance as a function
of oxygen (Figure 2) and iron (Figure 3) show definite trends. The figures
clearly reveal that the ratio of fluorine to oxygen [F/O] decreases from
the near-solar metallicity galactic stars, to lower metallicity LMC giants
and Arcturus (
Bootis).
Oxygen is produced in the supernova explosions of stars with masses greater
than ~8-10 Msun (SN II). In contrast, iron is mostly from lower
mass stars supernovae (SNI).

Figures 2 and 3. Logarithm abundance of fluorine, A(F),
plotted versus oxygen, A(O), and logarithm abundance of fluorine, A(F), plotted
versus iron, A(Fe). The colors of the symbols refer to the location of the
red giants: the Milky Way (blue squares), the LMC (green circles) and
Centauri (purple triangles). The encircled black dots correspond
to the solar abundance of fluorine

The main
mechanisms for producing fluorine in stars are: (1) neutrino-induced spallation
of a proton from Neon-20, referred to as the neutrino process; (2) synthesis
from helium capture by Nitrogen-14 during asymtoptic giant branch (AGB) thermal
pulses; and (3) production of Fluorine-19 in the cores of massive Wolf-Rayet
(W-R) stars. Because the
Centauri
stars show depletion of fluorine instead of enrichment, the authors easily
discard the AGB stars as an important cradle of fluorine. For Wolf-Rayet
stars to be major providers of fluorine, they need to undergo, more substantial
mass loss than currently accepted.

Although
a W-R source cannot be excluded at this stage, the best bet is the neutrino
process of spallation in supernovae of massive stars (SN II). Models of neutrino
nucleosynthesis predict that the [F/O] ratio declines steadily as oxygen
declines. This is shown very clearly in Figure 4.

The
Centauri
red giants stand out from this prediction and trend. We know that
Centauri
had 3 or 4 major isolated episodes of star formation, which ended several
billions years ago, giving the cluster its wide range of metal abundance.
In contrast, the Milky and the LMC are undergoing continuous star formation.

Taking into account the different
star-formation histories, the authors conclude that the dependence of
[F/O] on A(O) - as measured in Galactic LMC and
Centauri
red giant stars is best explained by the chemical evolution models using
the known neutrino spallation present in massive supernovae, and that these
explosions must be the principal source of fluorine-19.

Figure 4. The ratio of fluorine to oxygen abundance versus
oxygen in red giants stars located in the Milky Way, the LMC and
the rich globular cluster
Centauri.

Fluorine
is a relatively light element with a nucleus of nine protons and ten
neutrons. Its second shell has seven electrons and falls one short of
the eight electrons that would fully complete the second electron shell
- as it does with the next element Neon-20, which is a noble gas.
Because of its incomplete second electron shell, fluorine is the most
reactive and electronegative of all elements. It reacts with almost all
organic and inorganic substances. Water even burns in fluorine. Due to
its avidity for electron donors, it does not easily exist or survive on
its own.

The
French chemist Ferdinand Frederic Henri Moissant isolated fluorine in 1886,
and won the 1906 Nobel Prize for Chemistry for his impressive work. Because
of its unique, difficult and dangerous properties, uses for fluorine have
only recently been discovered. It is mainly used in sophisticated chemical
processes, such as the production of high temperature plastics and the isolation
of the isotopes of uranium. Fluorine also has other, more common uses. When
mixed in extremely small quantities in municipal water (one part per billion)
and in dental paste, fluorine effectively prevents tooth decay. Crystals
of calcium fluoride (CaF2), also known as fluorite and fluorspar,
are used to make lenses.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.